The instant invention is directed to an improved battery separator which exhibits very high degree of stability with respect to the conditions encountered in a battery system. In particular, the instant separator exhibits prolonged resistance to oxidative degradation. The present separator aids in providing a battery system capable of exhibiting extended activity.
Storage batteries are generally composed of at least one pair of electrodes of opposite polarity and an electrolyte. The battery may employ an acid or an alkaline electrolyte. Conventional acid batteries are exemplified by the lead-acid (sulfuric acid) batteries used in automobiles and the like. Alkaline batteries include secondary batteries such as nickel-zinc, nickel-cadmium, mercury-zinc and the like.
In addition to the electrodes and electrolyte, one of the recognized key components in a battery is the separator. Separators are elements located between electrodes to prevent direct contact between plates of opposite polarity while freely permitting electrolytic conduction. The separator must be porous to permit the electrolyte to be the sole internal conducting path between electrodes. Thus, although the separator itself must be substantially non-conducting, it must be capable of exhibiting low resistivity when in a battery system.
The separator is preferably in the form of a diaphragm or envelope (in which an electrode of at least one polarity is encased) which is microporous and has a high void volume. Such configurations permit the necessary free flow of electrolytic conductivity (low resistivity) while inhibiting active materials from passing through the separator causing unwanted bridging of plates of opposite polarity. Such contact may be due to imperfections in the plate structure or due to dendrite growth on the electrode during use or the like.
In addition to the above requirements the separator must be capable of being formed into a very thin sheet which is substantially inert to the environment established by the battery system. For example, it is well known that the battery forms an oxidative environment which causes degradation of materials used in forming conventional separators which, in turn, causes disintegration of the thin separator membrane. The ability to produce a separator membrane which is capable of withstanding the oxidative forces of the battery environment while fulfilling the other criteria of being microporous, having a high void volume, having a very thin cross-section, being inert to the battery components, exhibiting low electrical resistivity, and exhibiting high resistivity to passage of active material is highly desired.
U.S. Pat. No. 3,351,495 discloses a battery separator having a relatively low pore size and satisfactory electrical resistance characteristics made from a high molecular weight polyolefin having an average molecular weight of at least 300,000, a standard load melt index of substantially zero, and a reduced viscosity of not less than 4. The separator is manufactured by extruding the high molecular weight polyolefin in admixture with an inert filler and a plasticizer and then extracting the plasticizer by the use of a suitable solvent.
U.S. Pat. No. 4,024,323 discloses a variation to the '495 battery separator. The composition contains small amounts of a copolymer formed from an olefin or mixture of olefins (e.g., ethylene, propylene)and an ethylenically unsaturated carboxylic acid (e.g., acrylic or methacrylic acid) to aid in processability.
U.S. Pat. No. 4,287,276 is directed to the formation of a battery separator specifically useful in alkaline battery systems. The polyolefin matrix is highly filled with a particular class of fillers to form a microporous sheet having enhanced resistance to dendrite formation.
While the above separators have shown good stability, they do not exhibit the desired prolonged stability against oxidative forces encountered in the modern day, extended life batteries. Thus, these separators are known to degrade and permit shorting over a period of time. Extending the life of the battery can be achieved by using the presently described separator.